This invention relates to the production of a color developing record sheet for pressure-sensitive papers having a coating of metal-modified para-substituted phenolformaldehyde novolak resin as the color developers. More particularly, it relates to an improved method of grinding and stabilizing a particulate metal-modified novolak resin dispersion prior to preparation of the color developing coating composition by adding to the resin dispersion containing an anionic polymeric dispersing agent prior to grinding a small amount of an organophosphorus compound containing two or more phosphonic acid or alkali metal phosphonate groups per molecule.
Metal-modified novolaks are useful as developing agents for colorless color precursors such as Crystal Violet Lactone. Color producing systems of this type are useful in pressure-sensitive manifolding systems wherein a color developer, a color precursor and a common solvent for each are maintained in isolation on separate surfaces of a carrier. Conveniently, the color precursor and the solvent may be encapsulated in microcapsules of the type described in U.S. Pat. No. 2,800,457 and said microcapsules together with an adhesive may be applied as a coating to a web of paper. The color developer may be applied as a coating to a second web of paper, generally with a clay, such as kaolin clay or silton clay. When the two webs of paper are superimposed with their respective coatings in contact and subjected to localized pressure as by a typewriter key, the walls of the microcapsules in the area subjected to pressure are ruptured and the solution of color precursor and solvent originally contained in the ruptured capsules is transferred in a marking pattern to the paper surface carrying the color developer. Almost instantaneously a colored mark is formed on the surface carrying the color developer.
The use of finely ground dispersions of phenolformaldehyde novolak resins as the color developer in the preparation of pressure-sensitive record sheets is known. In the preparation of the ground dispersions of resin a number of dispersing agents for the resin have been used. U.S. Pat. No. 3,672,935 teaches the use of sodium silicate, gum arabic, casein, hydroxyethylcellulose or a latex, such as styrenebutadiene latex, for this purpose. Preparation of a ball milled resin dispersion containing 10% resin particles is disclosed.
The use of anionic polymeric type dispersing agents such as "Tamol 731" for dispersing a phenolformaldehyde resin during grinding in an attritor is disclosed in U.S. Pat. No. 3,516,845. The ground resin is compounded into an aqueous coating composition containing a soluble metal salt and the coating composition is applied to a paper and dried to form a pressure-sensitive record sheet. U.S. Pat. No. 3,732,120 discloses the use of a metal resinate, such as the reaction product of para-phenylphenolformaldehyde resin with zinc hydroxybenzoate as the color developer in pressure-sensitive record sheets. The metal resinate may be ground and applied as a coating to paper as taught by U.S. Pat. Nos. 3,455,721 or 3,672,935.
Using the dispersing agents suggested by the prior art, rheologically stable grinds of finely divided phenolformaldehyde novolak resin particles may be conveniently prepared and used to prepare stable color developing coating compositions. The preparation of rheologically stable grinds containing finely divided metal-modified phenolformaldehyde novolak resin particles is much more difficult. This is particularly true where the resin solids exceed 35% and temperatures above 68.degree. F are reached during the grinding period. Under these conditions, the dispersing agents normally used for dispersing the nonmodified phenolformaldehyde resin are not sufficient to give stable dispersions even when more than the normal amount of a dispersing agent is added. The dispersions do not remain fluid on standing after grinding and in extreme cases set up and cake during the after grinding period. This condition is aggravated by the long grinding periods encountered in the grinding of large volumes of resin dispersion for commercial production.
Using the process of this invention, the above difficulties can be overcome and rheologically stable grinds of metal-modified resin particles can be consistently produced in quantities required for commercial use. Such grinds are fluid enough to be handled in commercial equipment and will remain fluid on standing if there is a delay in the production process.